The long range goal of this project is to combine immunocytochemical methods with light and electron microscopy to study cellular mechanisms of myelin formation, breakdown, and regeneration. The main findings in current studies are: (l) Electron microscopic immunocytochemical observations using a different pre-treatment and another chromophore have confirmed localization of myelin-associated glycoprotein (MAG) on compact CNS myelin. (2) Examination of immuno-stained sections from additional acute and chronic cases of multiple sclerosis (MS) indicates that decreased anti-MAG immunoreactivity found earlier in normal appearing white matter around MS lesions is an infrequent finding when several antiserum concentrations are used in two different staining methods. (3) Antigen presenting cells (APCs), characterized by expression of cell surface markers (Ia antigens) an cytoplasmic enzymes (acid phosphatase), are present in early EAE lesions. They include small lymphocyte-like cells of either the T or B cell lineage and macrophages, the latter serving in the effector arm of this demyelinating process. (4) At birth, myelin basic proteins, P1 and P2, are only expressed in Schwann cells that are starting to form myelin sheaths. Anti-P1 stains all sheaths intensely and uniformly. Staining intensity with anti-P2 is variable and is highest in the largest sheaths. Factors responsible for this variation are being assessed.